Asphalt compositions that are used in paving applications include an aggregate component and an asphalt binder (also referred to as “liquid asphalt”). Hot mix asphalt paving compositions are made by heating and drying aggregate and mixing the aggregate with a proportioned amount of liquid asphalt. The liquid asphalt component binds the aggregate into a paving mixture that can be laid down and compacted to form roads, streets, airport run-ways, parking lots, walkways, etc.
Aggregates used in asphalt compositions for pavement applications generally have some porosity which tends to absorb asphalt into the pore structure. The amount or extend of this absorption is becoming more important as the sources for higher quality aggregates are used up and lower quality, higher porosity, aggregates must be used in their place. In addition, geographic proximity often predicates the utilization of marginal, more absorptive aggregates, since the alternative is to transport aggregate over distances which increases the cost to asphalt manufacturers.
In asphalt technology, absorption is a very complex subject. Although some absorption may lead to improved strength in compacted mixtures due to mechanical interlocking, the portion of the asphalt which is absorbed is not available for binding the aggregate particles together in the asphalt paving mix. In addition, when porous aggregates absorb asphalt components slowly over time, undesirable voids can be produced in applications such as pavements. Furthermore, if the porous aggregate selectively absorbs components of the asphalt, the nonabsorbed asphalt may have different physical, rheological, and chemical properties than the original asphalt. In such a case, the nonabsorbed bulk asphalt (effective asphalt) may be the weak link because selective absorption may promote extraction of certain asphalt fractions such as the polar compounds, aromatics, or saturates from the maltenes, leaving the bulk phase rich in such fractions as asphaltenes. This component segregation may lead to less temperature susceptible bulk phases which may in turn be more susceptible to distresses such as, low temperature cracking, fatigue and moisture damage.
The most common method of attempting to deal with absorptive aggregates is to increase the asphalt content of the mix; this solves the problem sometimes, but not always. If the absorption is fast enough and is essentially completed during the mix design procedure, this solution is adequate. If, on the other hand, the absorption is slow, the need for additional asphalt content will not be detected in the mix design, resulting in mixes which eventually have too little asphalt, and fail. Adding extra asphalt to the slow absorbing aggregate also causes problems because the mix is over-asphalted when placed and will be tender and difficult to compact. The mix may also flush initially in the pavement.
While there are several methods that have been proposed for determining the absorption capacity of aggregates, no standard test method has been developed to evaluate, describe and specify the absorptive characteristic of an aggregate with respect to asphalt.
Nevertheless, within the asphalt industry porous aggregates have been recognized and generally identified by source or type. For example, Florida lime rock and southern Indiana lime rock are known identified sources of porous aggregates which are not generally suitable for use in asphalt compositions. In fact, in the case of Florida lime rock, rather than use this aggregate in local asphalt applications, other less porous aggregate is transported to Florida from Canada for use in asphalt applications. An example of a recognized type of porous aggregate is blast furnace slag. More generally, aggregate materials are classified as natural and artificial, with natural aggregates including those from natural sources such as gravels and natural sand, and crushed stone, and artificial aggregate including blast furnace slag, steel slag, wet-bottom boiler slag, and lightweight aggregate. There are both natural and artificial porous aggregate materials.
The present invention provides a method of incorporating porous aggregate materials into asphalt compositions in such a manner that reduces the overall amount of liquid asphalt in the resulting compositions and avoids problems associated with selective absorption.